The present invention relates to the Y/C separation of a composite color picture signal used in a television receiver, a video tape recorder (VTR), and others.
Recently, in order to improve not only the picture quality of a high-resolution VTR such as a VTR of the S-VHS type but also the picture quality of a home VTR and a television receiver, a comb filter which utilizes vertical correlation to obtain a high resolution, has been generally used as the Y/C separator of a composite color picture signal (hereinafter referred to as "composite color video signal").
Further teletext which is one of new media utilizing the electromagnetic wave for television, has been put to practical use. A teletext signal as well as reference signals for broadcasting such as a ghost cancel reference (GCR) signal, a multiburst signal and a modulated sinusoidal wave signal is sent in the vertical retrace interval of a television signal. Further, in the vertical retrace interval of a video disk of the laser type, reference signals such as a color bar signal are superposed on a video signal. When the television or video signal is reproduced, a transmission system can be checked by observing the waveforms of the reference signals inserted in a broadcasting wave or laser vision.
Signals on a plurality of lines in the vertical retrace interval are independent of each other, and thus correlation does not exist between the lines. Accordingly, when the Y/C separation of the reference signals are carried out by means of the above-mentioned comb filter, the reference signals will be greatly distorted.
For example, in a case where the picture recording of the teletext is made, even when a VTR of the S-VHS type having a sufficient band-width is used, an error will be frequently produced in the output of a teletext decorder applied with a recorded and reproduced video signal. Further, reference signals which are superposed on a video signal in the vertical retrace interval thereof cannot be used to check a reproducing system including a Y/C separator, because the reference signals are distorted by Y/C separation. A method of making possible the picture recording of the teletext is disclosed in a Japanese patent application JP-A-hei 2-71,692. In this method, a comb filter used for Y/C separation is by-passed in the vertical retrace interval.
The above-mentioned conventional Y/C separator will be explained below with reference to FIG. 4.
FIG. 4 is a block diagram showing the circuit configuration of the conventional Y/C separator. In FIG. 4, reference numeral 1 designates an input terminal applied with a composite signal, 2 a comb filter which is made up of a 1-H delay circuit 21, an adder 22, a subtracter 23 and 1/2 attenuators 24a and 24b, and is used in an NTSC color television system, 3 a synchronous separation circuit for generating a pulse in the vertical retrace interval, 4 a switching circuit for performing a switching operation in response to the pulse from the synchronous separation circuit 3, 5 an output terminal for delivering a luminance signal Y, and 6 an output terminal for delivering a color signal C.
The operation of the Y/C separator having the above circuit configuration will be explained.
An inputted video signal V.sub.in is applied to the comb filter 2. In the NTSC color television system, the luminance signal Y of a first signal is in phase with the luminance signal Y of a second signal which is obtained by delaying the first signal by a time corresponding to one horizontal scanning line, but the color signal C of the first signal is 180.degree. out of phase with the color signal C of the second signal. Accordingly, on the basis of the vertical correlation of a general video signal, the comb filter 2 can detect the luminance signal Y from the sum of the first and second signals and can detect the color signal C from the difference between the first signal and the second signal. The movable contacts of the switching circuit 4 are usually put on the a-side, and thus the luminance signal Y and color signal C from the comb filter 2 are sent to the output terminals 5 and 6, respectively.
When a vertical retrace interval is started, the synchronous separation circuit 3 generates a changeover pulse, and thus the movable contacts of the switching circuit 4 are put on the b-side. As a result, in the vertical retrace interval, the input signal V.sub.in is sent to the output terminal 5, as it is, to be used as the outputted luminance signal Y out, and the outputted color signal C.sub.out becomes zero. Accordingly, a teletext signal which is superposed on the video signal in the vertical retrace interval thereof, is delivered as the outputted luminance signal Y.sub.out, without being distorted.
According to the Y/C separator of FIG. 4, however, there arises a problem that a color signal which is superposed on the video signal in the vertical retrace interval thereof is made monochrome.
Accordingly, it has been earnestly desired to develop a Y/C separator which can carry out the Y/C separation of various signals superposed on a video signal in the vertical retrace interval thereof other than the teletext signal, while making the distortion of the above signals as small as possible.